CacheViewer for FF updated versions Rated 5 out of 5 stars
Click "Add to Firefox", right mouse click -> "Save Link As..." in "Install Anyway" when it appears.
Extract to a empty folder, edit "install.rfd" in notepad and change the max version to " * " only, making it look like "<em:maxVersion>*</em:maxVersion>".
Compress to file in ZIP format, change the .zip extension to .xpi and drag the file into/open it with FF.
This way the extension won't be disabled every time you update your FF version, but you have to keep an eye out to see if it continues to work from version to version.